Fix gemini candidate starvation

TODO.md

@@ -655,6 +655,30 @@
 - If behavior in the browser does not match the latest backend/frontend code, the first assumption should be stale frontend assets until proven otherwise
 
 ## Recent Change Log
+- Date: `2026-03-17`
+- What changed:
+  - Fixed a search-budget regression where source collection could consume the full `SearchService` deadline and leave no time for Envato / Artgrid enrichment, causing Gemini to see only missing or low-value visuals.
+  - Split the search-service deadline into:
+    - collector deadline
+    - enrichment deadline with an explicit reserved window
+  - Added unit coverage for the new deadline split behavior.
+  - Stopped frontend preview-probe fallback from calling `/api/download/preview` for Artgrid items that do not already have a provider preview URL, so unsupported `yt-dlp` Artgrid probe errors no longer fire just from opening or hovering those results.
+- Why it changed:
+  - The user-provided log `ai-media-hub-2026-03-17T07-01-21-282Z.log` showed:
+    - `search_service:deadline_reached`
+    - immediate `search_service:enrich_start` -> `search_service:enrich_complete`
+    - `withPreview: 0`
+    - `withLowValueThumbnail: 12`
+    - repeated `candidate has no thumbnail or preview video`
+    - final warning `gemini vision returned no candidate evaluations`
+  - The same log also showed Artgrid preview probe failures from `yt-dlp` returning `Unsupported URL`, which were not helping user-facing preview behavior.
+- How it was verified:
+  - `pwsh -NoProfile -File scripts/selftest.ps1`
+  - added Go tests for the search/enrichment deadline split helper
+- What is still risky or incomplete:
+  - This preserves time for enrichment, but it does not guarantee that every live Envato / Artgrid page yields a usable preview URL.
+  - Artgrid still depends on backend-enriched provider preview URLs for true video preview; if no provider preview is discovered, the UI will still fall back to thumbnail-only rendering.
+
 - Date: `2026-03-17`
 - What changed:
   - Added repo-local Windows 11 PowerShell workflows: